The present invention relates to an image processing method and apparatus, a recording medium and an imaging apparatus, and more particularly to an image processing method and apparatus for determining the variance of noise of an image, a medium for recording a program for a computer to perform such an image processing function, and an imaging apparatus comprising such an image processing apparatus.
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, i.e., a space in which a static magnetic field is generated; a gradient magnetic field and a high frequency magnetic field are applied to generate a magnetic resonance signal inside the object; and a tomographic image is produced (reconstructed) based on the received signal.
In order for an observer to view a fine structure in the tomographic image in more detail, filtering is performed to remove noise in the image. Although the filtering is basically low-pass filtering, since the sharpness of the image is lowered by using the low-pass filtering alone, filtering with an additional process for preserving the sharpness is employed.
Since the filtering in combination with the sharpness preserving process, however, has a side effect that an incidental texture created by noise is enhanced and an anatomically meaningless structure (false structure) is generated, it is necessary to know whether each local portion of the image contains only noise or contains a true structure as well, in order to exclude the portion containing only noise from the sharpness preservation process.
To discriminate noise from a true structure, a method is contemplated involving previously evaluating the variance of noise contained in an image, and identifying a local portion as noise if the variance of pixel values in the local portion and the variance of noise have no significant difference, or otherwise, identifying the local portion as a structure. However, it is impossible to accurately separate only noise from an image containing noise mixed with an image signal, and hence, the variance of noise could not be evaluated.
Therefore, an object of the present invention is to provide an image processing method and apparatus for determining the variance of image noise, a medium for recording a program for a computer to perform such an image processing function, and an imaging apparatus comprising such an image processing apparatus.
(1) The present invention, in accordance with one aspect thereof for solving the aforementioned problem, is an image processing method characterized in that the method comprises: determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; obtaining a histogram of said residual sums of squares; and determining a variance of noise based on the residual sum of squares that gives a peak of said histogram.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, and the variance of noise is determined based on the residual sum of squares that gives a peak of a histogram of the residual sums of squares, the variance of image noise can be accurately determined.
(2) The present invention, in accordance with another aspect thereof for solving the aforementioned problem, is an image processing method characterized in that the method comprises: determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; obtaining a histogram of said residual sums of squares; fitting said histogram to a function; and determining a variance of noise based on a variable that gives a peak value of said fitted function.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, a histogram thereof is fitted to a function, and the variance of noise is determined based on a variable that gives a peak value of the function, the variance of image noise can be more accurately determined.
(3) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding (2), characterized in that said function is a Gaussian distribution function.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a substantial portion of an image.
(4) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding (2), characterized in that said function is a Rayleigh distribution function.
According to the invention in this aspect, since the histogram is fitted to a Rayleigh distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a background portion of an image.
(5) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding (2), characterized in that said function comprises Gaussian and Rayleigh distribution functions.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function and a Rayleigh distribution function, variables that give peaks of the functions can be accurately determined for image noise in substantial and background portions of an image.
(6) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding any one of (1)-(5), characterized in that the pixel value in said image is the absolute value of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the absolute value of a complex number.
(7) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding any one of (1)-(3), characterized in that the pixel value in said image is the real part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the real part of a complex number.
(8) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding any one of (1)-(3), characterized in that the pixel value in said image is the imaginary part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the imaginary part of a complex number.
(9) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing method as described regarding any one of (1)-(8), characterized in that said image is one captured by using magnetic resonance.
According to the invention in this aspect, the variance of noise can be determined for an image captured using magnetic resonance.
(10) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus characterized in that the apparatus comprises: residual sum of squares calculating means for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; histogram calculating means for obtaining a histogram of said residual sums of squares; and noise variance calculating means for determining a variance of noise based on the residual sum of squares that gives a peak of said histogram.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, and the variance of noise is determined based on the residual sum of squares that gives a peak of a histogram of the residual sums of squares, the variance of image noise can be accurately determined.
(11) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is an image processing apparatus characterized in that the apparatus comprises: residual sum of squares calculating means for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; histogram calculating means for obtaining a histogram of said residual sums of squares; fitting means for fitting said histogram to a function; and noise variance calculating means for determining a variance of noise based on a variable that gives a peak value of said fitted function.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, a histogram thereof is fitted to a function, and the variance of noise is determined based on a variable that gives a peak value of the function, the variance of image noise can be more accurately determined.
(12) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding (11), characterized in that said function is a Gaussian distribution function.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a substantial portion of an image.
(13) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding (11), characterized in that said function is a Rayleigh distribution function.
According to the invention in this aspect, since the histogram is fitted to a Rayleigh distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a background portion of an image.
(14) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding (11), characterized in that said function comprises Gaussian and Rayleigh distribution functions.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function and a Rayleigh distribution function, variables that give peaks of the functions can be accurately determined for image noise in substantial and background portions of an image.
(15) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding any one of (10)-(14), characterized in that the pixel value in said image is the absolute value of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the absolute value of a complex number.
(16) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding any one of (10)-(12), characterized in that the pixel value in said image is the real part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the real part of a complex number.
(17) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding any one of (10)-(12), characterized in that the pixel value in said image is the imaginary part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the imaginary part of a complex number.
(18) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding any one of (10)-(17), characterized in that said image is one captured by using magnetic resonance.
According to the invention in this aspect, the variance of noise can be determined for an image captured using magnetic resonance.
(19) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is a recording medium characterized in that the medium records in a computer-readable manner a program for a computer to perform: a residual sum of squares calculating function for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; a histogram calculating function for obtaining a histogram of said residual sums of squares; and a noise variance calculating function for determining a variance of noise based on the residual sum of squares that gives a peak of said histogram.
According to the invention in this aspect, since a program recorded on the recording medium makes a computer perform the functions of determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image, and determining the variance of noise based on the residual sum of squares that gives a peak of a histogram of the residual sums of squares, the variance of image noise can be accurately determined.
(20) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is a recording medium characterized in that the medium records in a computer-readable manner a program for a computer to perform: a residual sum of squares calculating function for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; a histogram calculating function for obtaining a histogram of said residual sums of squares; a fitting function for fitting said histogram to a function; and a noise variance calculating function for determining a variance of noise based on a variable that gives a peak value of said fitted function.
According to the invention in this aspect, since a program recorded on the recording medium makes a computer perform the functions of determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image, and fitting a histogram thereof to a function, and determining the variance of noise based on a variable that gives a peak value of the function, the variance of image noise can be more accurately determined.
(21) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding (20), characterized in that said function is a Gaussian distribution function.
According to the invention in this aspect, since a program recorded on the recording medium makes a computer perform the function of fitting the histogram to a Gaussian distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a substantial portion of an image.
(22) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding (20), characterized in that said function is a Rayleigh distribution function.
According to the invention in this aspect, since a program recorded on the recording medium makes a computer perform the function of fitting the histogram to a Rayleigh distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a background portion of an image.
(23) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding (20), characterized in that said function comprises Gaussian and Rayleigh distribution functions.
According to the invention in this aspect, since a program recorded on the recording medium makes a computer perform the function of fitting the histogram to a Gaussian distribution function and a Rayleigh distribution function, variables that give peaks of the functions can be accurately determined for image noise in substantial and background portions of an image.
(24) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding any one of (19)-(23), characterized in that the pixel value in said image is the absolute value of a complex number.
According to the invention in this aspect, a program recorded on the recording medium can make a computer perform the function of determining the variance of noise for an image with pixel values of the absolute value of a complex number.
(25) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding any one of (19)-(21), characterized in that the pixel value in said image is the real part of a complex number.
According to the invention in this aspect, a program recorded on the recording medium can make a computer perform the function of determining the variance of noise for an image with pixel values of the real part of a complex number.
(26) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding any one of (19)-(21), characterized in that the pixel value in said image is the imaginary part of a complex number.
According to the invention in this aspect, a program recorded on the recording medium can make a computer perform the function of determining the variance of noise for an image with pixel values of the imaginary part of a complex number.
(27) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the recording medium as described regarding any one of (19)-(26), characterized in that said image is one captured by using magnetic resonance.
According to the invention in this aspect, a program recorded on the recording medium can make a computer perform the function of determining the variance of noise for an image captured using magnetic resonance.
(28) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is an imaging apparatus for producing an image based on a signal collected from an object, characterized in that the apparatus comprises: residual sum of squares calculating means for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; histogram calculating means for obtaining a histogram of said residual sums of squares; and noise variance calculating means for determining a variance of noise based on the residual sum of squares that gives a peak of said histogram.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, and the variance of noise is determined based on the residual sum of squares that gives a peak of a histogram of the residual sums of squares, the variance of image noise can be accurately determined.
(29) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is an imaging apparatus for producing an image based on a signal collected from an object, characterized in that the apparatus comprises: residual sum of squares calculating means for determining a residual sum of squares of pixel values for each of a plurality of local regions defined over an entire image; histogram calculating means for obtaining a histogram of said residual sums of squares; fitting means for fitting said histogram to a function; and noise variance calculating means for determining a variance of noise based on a variable that gives a peak value of said fitted function.
According to the invention in this aspect, since a residual sum of squares of pixel values is determined for each of a plurality of local regions defined over an entire image, a histogram thereof is fitted to a function, and the variance of noise is determined based on a variable that gives a peak value of the function, the variance of image noise can be more accurately determined.
(30) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding (29), characterized in that said function is a Gaussian distribution function.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a substantial portion of an image.
(31) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding (29), characterized in that said function is a Rayleigh distribution function.
According to the invention in this aspect, since the histogram is fitted to a Rayleigh distribution function, a variable that gives a peak of the function can be accurately determined for image noise in a background portion of an image.
(32) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding (29), characterized in that said function comprises Gaussian and Rayleigh distribution functions.
According to the invention in this aspect, since the histogram is fitted to a Gaussian distribution function and a Rayleigh distribution function, variables that give peaks of the functions can be accurately determined for image noise in substantial and background portions of an image.
(33) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding any one of (28)-(32), characterized in that the pixel value in said image is the absolute value of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the absolute value of a complex number.
(34) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding any one of (28)-(30), characterized in that the pixel value in said image is the real part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the real part of a complex number.
(35) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the imaging apparatus as described regarding any one of (28)-(30), characterized in that the pixel value in said image is the imaginary part of a complex number.
According to the invention in this aspect, the variance of noise can be determined for an image with pixel values of the imaginary part of a complex number.
(36) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is the image processing apparatus as described regarding any one of (28)-(35), characterized in that said signal is a magnetic resonance signal.
According to the invention in this aspect, the variance of noise can be determined for an image captured using magnetic resonance.
Therefore, the present invention can provide an image processing method and apparatus for determining the variance of noise of an image, a medium for recording a program for a computer to perform such an image processing function, and an imaging apparatus comprising such an image processing apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.